Communicating over coaxial cable networks

ABSTRACT

A method for communicating over a coaxial cable network is described. The method includes identifying at least one port in the coaxial cable network that provides high mutual isolation among nodes of the coaxial cable network when the port is terminated with an impedance that matches a characteristic impedance of coaxial cable in the coaxial cable network. The method also includes terminating the identified port with an impedance that is substantially mismatched with the characteristic impedance of the coaxial cable, and transmitting a signal from a first node in the network to a second node in the network.

TECHNICAL FIELD

This invention relates to communicating over coaxial cable networks.

BACKGROUND

Coaxial cable transmission lines can be used to route radio frequency(rf) signals throughout a home. The characteristics of a coaxial cabledetermine what maximum frequency the cable will support for high quality(e.g., high signal-to-noise ratio) transmission of analog or digitalsignals. Older cable existing in many homes may support high qualitytransmission of signals up to around 900 MHz. Other types of cable(e.g., cable used for satellite television signals) may support higherfrequencies up to around 1700 MHz. The frequency limit also determinesthe maximum data rate limits for digital signals (e.g., digital video orinternet protocol (IP) data packets).

A cable signal typically enters a home over a single source port andfrom there is distributed throughout the home. A distribution network ofcoaxial cable is typically formed by connecting cables to splitters thatpassively couple an incoming signal to two or more output ports. Thisnetwork typically has a tree topology in which information flowsdownstream from the source (at the “root” of the tree) to eachterminating device such as a television, set top box, or cable modem(the “leaves” of the tree). In some cases (e.g., for a cable modem orinteractive television service) information also flows upstream from aterminating device to the source port.

SUMMARY

In a first aspect, the invention features a method for communicatingover a coaxial cable network. The method includes identifying at leastone port in the coaxial cable network that provides high mutualisolation among nodes of the coaxial cable network when the port isterminated with an impedance that matches a characteristic impedance ofcoaxial cable in the coaxial cable network. The method also includesterminating the identified port with an impedance that is substantiallymismatched with the characteristic impedance of the coaxial cable, andtransmitting a signal from a first node in the network to a second nodein the network.

Preferred implementations of this aspect of the invention mayincorporate one or more of the following:

The identified port includes an input port to a splitter having at leasttwo output ports that are mutually isolated when the input port isterminated with an impedance that matches the characteristic impedanceof the coaxial cable.

The splitter includes a hybrid splitter.

The identified port is positioned in the network to distribute anincoming signal from a source to terminal nodes of the coaxial cablenetwork.

The source is a cable television feeder cable, a terrestrial antenna, ora satellite dish.

Terminating the identified port with the mismatched impedance includescoupling the incoming signal from the source to the identified port withan output impedance that is substantially mismatched with thecharacteristic impedance of the coaxial cable.

Terminating the identified port with the mismatched impedance includesuncoupling the source from the identified port.

Transmitting the signal from the first node to the second node includescoupling a signal from the first node with an output impedance that issubstantially mismatched with the characteristic impedance of thecoaxial cable.

The output impedance is substantially smaller than the characteristicimpedance of the coaxial cable.

The output impedance is smaller than about 10% of the characteristicimpedance of the coaxial cable.

Transmitting the signal from the first node to the second node includescoupling a signal to the second node with an input impedance that isthat is substantially mismatched with the characteristic impedance ofthe coaxial cable.

The input impedance is substantially larger than the characteristicimpedance of the coaxial cable.

The output impedance is larger than about 300% of the characteristicimpedance of the coaxial cable.

The coaxial cable network has a tree topology with the identified portat the root of the tree.

In a second aspect, the invention features a coaxial cable network. Thenetwork includes a source port providing an input signal, a coaxialcable coupling the source port to a first splitter, and a plurality ofcoaxial cables providing an interface for nodes of the network. At leastsome of the coaxial cables are coupled to the source port over a paththat includes at least one splitter. At least one splitter port provideshigh mutual isolation among nodes of the coaxial cable network when thesplitter port is terminated with an impedance that matches acharacteristic impedance of coaxial cable in the coaxial cable network.The splitter port is terminated with an impedance that is substantiallymismatched with the characteristic impedance of the coaxial cable.

Preferred implementations of this aspect of the invention mayincorporate one or more of the following:

The splitter port includes an input port to the first splitter, thefirst splitter having at least two output ports that are mutuallyisolated when the input port is terminated with an impedance thatmatches the characteristic impedance of the coaxial cable.

The coaxial cable network has a tree topology with the input port to thefirst splitter at the root of the tree.

The first splitter is positioned in the network to distribute theincoming signal from the source port to terminal nodes of the coaxialcable network.

The coaxial cable network further includes a node coupled to a coaxialcable interface with an output impedance that is substantiallymismatched with the characteristic impedance of the coaxial cable.

The coaxial cable network further includes a node coupled to a coaxialcable interface with an input impedance that is that is substantiallymismatched with the characteristic impedance of the coaxial cable.

Among the many advantages of the invention (some of which may beachieved only in some of its various aspects and implementations) arethe following.

Mismatching the impedance at one or more splitters in a coaxial cablenetwork reduces attenuation due to isolation between nodes in thenetwork which increases the data rate and reliability of communicationbetween nodes. Placing an impedance mismatched gateway device at theroot node of a tree network enables communication among the leaf nodeswhile maintaining the ability to distribute a source signal to the leafnodes. Coupling transmitting devices to a coaxial cable network with alow output impedance and coupling receiving devices to the coaxial cablenetwork with a high input impedance provides low-loss communication overa wide range of network characteristics including, for example, variouscable lengths and various numbers of splitters.

Other features and advantages of the invention will be found in thedetailed description, drawings, and claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of a coaxial cable network.

FIG. 2A is a circuit diagram of a source circuit element.

FIG. 2B is a circuit diagram of a load circuit element.

FIG. 2C is a circuit diagram of transmitting device connected to areceiving device by a transmission line.

FIG. 2D is a circuit diagram of a hybrid splitter.

FIGS. 2E and 2F are diagrams of equivalent circuits modeling the stateof the hybrid splitter.

FIGS. 3A-3D are plots of transfer responses for a simulation of acoaxial cable network.

FIG. 4 is a schematic diagram of a communication system.

FIG. 5A is a schematic diagram of an analog front end module.

FIG. 5B is a circuit diagram of a coupling module.

FIG. 6 is a circuit diagram of a passive bridge.

FIG. 7 is a representation of a passive bridge.

FIG. 8 is a circuit diagram of a hybrid coupler.

FIG. 9 is a plan view of a residential test site.

FIG. 10 is a schematic diagram of a test setup used to perform thetransfer response test measurements.

FIGS. 11A and 11B are grids showing measurement results.

DETAILED DESCRIPTION

There are a great many possible implementations of the invention, toomany to describe herein. Some possible implementations that arepresently preferred are described below. It cannot be emphasized toostrongly, however, that these are descriptions of implementations of theinvention, and not descriptions of the invention, which is not limitedto the detailed implementations described in this section but isdescribed in broader terms in the claims.

System Overview

Referring to FIG. 1, a coaxial cable network 100 in a home includes asource port 104 for a source cable 106 that carries an incoming signalfrom a source 108 outside of the home. For example, the source 108 canbe wired source that provides a signal over a distribution network thatis fed from a head-end at a cable television distribution center todistribution coaxial cables (e.g., “trunk” or “feeder” cables).Alternatively, the source 108 can be wireless source such as aterrestrial antenna that receives a signal from a broadcast tower, or asatellite dish that receives a signal from a satellite.

The coaxial cable network 100 distributes a signal throughout the homefrom the source port 104, through a gateway device 102, to standarddevices 110 (e.g., cable or satellite television set top boxes) andnetwork devices 112 over coaxial cable 111 (e.g., RG6 type coaxialcable). The coaxial cable network 100 includes splitters that splitinput signal power among multiple output ports. In this exemplarynetwork 100, the first splitter 113 is a 4-port, 3-way splitter thatdivides the signal at the input port evenly among three output ports.Alternatively, some splitters provide more power to some ports than toothers. These uneven splitters can be used to ensure certain devices(e.g., cable modems) have a large enough signal, or to provide morepower to ports that will undergo further splitting to feed moredownstream terminal nodes or “leaf” nodes. The coaxial cable network 100also includes 3-port, 2-way splitters 114 that divide the signal at theinput port evenly between two output ports. The coaxial cable network100 includes a bridge device 116 that couples the network 100 to asecondary network 120 such as a power line communication network thatuses existing AC wiring in a house to exchange information between nodesthat interface with AC outlets.

The gateway device 102 enables the network devices 112 to communicatewith each other, while continuing to distribute the incoming signal fromthe source port 104 to the standard devices 110. In a typical cabledistribution network in a home, to reduce interference on the network,the splitters 113 and 114 provide high isolation among the output portssuch that a signal entering one output port of the splitter is coupledto the input port and effectively cancelled at the other output port(s).For example, a “hybrid splitter” (or “magic tee” splitter) is typicallydesigned to provide high isolation among output ports for a givenimpedance at the input port. As explained in more detail below, theimpedance at which this high isolation occurs is designed to match thecharacteristic impedance of a given type of coaxial cable. Isolation of20 to 60 dB is typical in practice depending on the precision of thecomponents. This high attenuation would reduce the signal-to-noise ratio(SNR) which would in turn reduce the channel capacity (data rate).

The gateway device 102 terminates the “root” port 122 of the coaxialcable network with an impedance that is mismatched with thecharacteristic impedance designed to provide high isolation. Asdescribed in more detail below, this mismatch “propagates” throughoutthe tree-structured network 100 to mismatch the input ports of the othersplitters enabling any node in the network to communicate with any othernode without suffering drastic reduction in SNR due to high isolation.Alternatively, the root port 122 can be disconnected from the sourceport 104 to mismatch the network 100 without the need for a gatewaydevice 102 (though this configuration would no longer distribute theincoming signal to the standard devices 110).

The standard devices 110 are configured to receive the signal from thesource port 104 (and optionally to transmit signals to the source port104) without interfering with each other. In particular, the standarddevices 110 terminate the coaxial cables 111 with the characteristicimpedance Z₀ of the cable 111 (e.g., for RG6 coaxial cable Z₀=75 Ohms).Even though the splitters no longer provide high isolation, thisimpedance matching effectively eliminates reflections of a signal fromthe input of one standard device 110 that could interfere with anotherstandard device 110.

The coaxial cable network 100 is coupled to network devices 112 that areconfigured to transmit signals to and receive signals from other networkdevices 112 coupled to the network 100. The network devices 112 arehalf-duplex devices that switch between a transmit state and a receivestate (the default state). The network devices 112 can use any of avariety of types of medium access control (MAC) protocols such as acarrier sense multiple access with collision avoidance (CSMA/CA)protocol to coordinate communication over the network 100. The networkdevices 112 can optionally terminate the coaxial cables 111 with animpedance that depends on whether the device is in the transmit state orthe receive state to improve signal characteristics such assignal-to-noise ratio (SNR), as described in more detail below.

The standard devices 110 and the network devices 112 communicate overdifferent frequency bands using filters to reduce any potentialinterference between the standard and network devices. For example, inone scenario the standard devices receive a signal in the 50 to 800 MHzrange and the network devices communicate in the 2 to 28 MHz range. Eachnetwork device 112 includes a 35 MHz low-pass filter (LPF) to interfacewith the network 100, and each standard device includes a 50 MHzhigh-pass filter (HPF) to interface with the network 100. Thecombination of the LPFs and HPFs reduce potential interference caused bysignal energy transmitted from or reflected from unmatched networkdevices 112.

Alternatively, all of the devices coupled to the output ports of thesplitters can be network devices 112, in which case, the filters are notnecessarily used.

Impedance Matching and Mismatching

The characteristics of impedance matching and mismatching can beunderstood by examining simplified circuit models of the coaxial cablenetwork 100 and the various devices coupled to the network acting astransmitters and/or receivers. Referring to FIG. 2A, when a device istransmitting a signal into a port of the coaxial cable network, thatdevice can be modeled as a “source” circuit element 200 having a voltagesource 202 that provides a source voltage signal V_(S)(t) in series withan impedance Z_(out) that represents the output impedance of the device.Referring to FIG. 2B, when a device is receiving a signal over a coaxialcable of the network 100, that device can be modeled as a “load” circuitelement 204 having an impedance Z_(in) that represents the inputimpedance of the device.

Referring to FIG. 2C, a transmitting device 210, represented by sourcecircuit element 200, is connected to a receiving device 212, representedby load circuit element 204, over a coaxial cable modeled as atransmission line 220 having a length l. The voltage signal V_(R)(t)that is received by the receiving device 212 is a function of the sourcevoltage signal V_(S)(t), but also depends on the impedances Z_(out) andZ_(in) and the characteristic impedance Z₀ of the transmission line 220.In general, to the extent that either Z_(out) or Z_(in) differs from thecharacteristic impedance Z₀, there will be reflections that propagatebetween the input port 222 and output port 224 of the transmission line220 causing distortions in the received voltage signal V_(R)(t)including frequency selective distortions and time distortions such asmultiple delayed versions of a signal arriving over a time period called“delay spread.” For a transmission line terminated with a “mismatched”load impedance at the output port 224 that differs from thecharacteristic impedance Z₀, the effective impedance seen at the inputport 222 is transformed by the transmission line (e.g., as given by aSmith Chart). For example, depending on the length l, a real loadimpedance (i.e., resistance) of R_(L) can be transformed to an inductiveor capacitive impedance or to a real impedance of Z² ₀/R_(L) (when l isa quarter wavelength). However, a mismatched impedance remainsmismatched for any length l or signal frequency. The expected behaviorof a given network can be predicted according to standard transmissionline theory where each section of coaxial cable in the network ismodeled as a transmission line.

Typically, the input and output impedances of devices coupled to thenetwork 100 are “matched” to the characteristic impedance of the coaxialcable (i.e., Z_(out)=Z₀ and Z_(in)=Z₀). In this matched case, thereflections are eliminated (or in practice, due to the limited precisionof the components, at least greatly reduced) and the received voltagesignal V_(R)(t) is related to the source voltage signal as V_(R)(t)=0.5V_(S)(t−l/v), where v is the propagation velocity of the transmissionline (typically around 0.6-0.8 times the speed of light for coaxialcables). In practice, for a matched transmission line the receivedvoltage signal is a scaled and delayed version of the source voltagesignal over a wide range of frequencies, and does not suffer thefrequency distortions or delay spread of the mismatched transmissionline.

A typical splitter is designed to terminate a coaxial cable coupled toits input port with a matched impedance when the output ports of thesplitter are terminated with matched load impedances. The typicalsplitter is also designed to provide a matched output impedance to eachload. Thus, the splitter is designed to preserve the impedance matchingcharacteristics of a network. In addition to preserving impedancematching, a typical splitter is designed to provide high isolation amongits output ports.

Referring to FIG. 2D, one example of a 3-port, 2-way splitter 114 havinghigh isolation among output ports is a hybrid splitter modeled as acircuit 230 that has a single input port 231 and two output ports 232and 233. The input port 231 is coupled to a 2:1 impedance transformer234 that transforms the output impedance of a device coupled to theinput port 231 by a factor of ½ (e.g., a transformer with a turns ratioof √2:1 yields an impedance ratio of 2:1). The three ports are connectedto a center-tap autotransformer 236 which couples signals among some ofthe ports under certain conditions. A shunt resistor 238 is connected tothe autotransformer 236 to establish conditions such that the outputports 232 and 233 can be mutually isolated.

Due to the symmetry of the circuit 230, an input signal at port 231 isevenly divided between ports 232 and 233. However, when a signal isapplied to the output port 232, the circuit 230 sets a voltage at theother output port 233 based on the impedance at the input port 231.Referring to FIG. 2E, a source 240 coupled to the output port 232 seesthe equivalent circuit 242 due to the impedance transformationproperties of the autotransformer 236. In particular, theautotransformer 236 transforms the impedance 2Z₀ of the shunt resistor238 by a factor of ¼ (since the turns ratio is ½) to a value of Z₀/2.Similarly, the impedance transformer 234 transforms the impedance Z₁ atthe input port 231 by a factor of ½ to a value of Z₁/2. Thus, the source240 sees the equivalent circuit 244 (FIG. 2F) and applies a sourcevoltage V_(S)(t) across three impedances: the output impedance Z_(out)an impedance Z₀/2 due to the splitter circuit 230, and an impedance Z₁/2due to the termination of input port 231.

The properties of autotransformer 236 ensure that the voltage dropV_(x)(t) across the top half of the autotransformer 236 is the same asthe voltage drop across the bottom half of the autotransformer. When theimpedance Z₁ at the input port 231 is equal to the characteristicimpedance Z₀, the voltage drop V_(x)(t) across the top half of theautotransformer 236 is equal to the voltage drop from the mid-point ofthe autotransformer 236 to ground. Therefore, in this “matched inputport” case, the voltage drop V_(x)(t) across the bottom half of theautotransformer 236 sets the voltage at the output port 233 to ground,regardless of the value of the source voltage V_(S)(t) or source outputimpedance Z_(out). In this case, all of the power delivered into outputport 232 is coupled to the input port 231 (neglecting internal splitterlosses). This ideal model exhibits complete isolation, however, inpractice hybrid splitters suffer from leakage current and leakageinductance such that isolation of 20 to 60 dB is possible over anoperating bandwidth, depending on the precision of the splittercomponents.

When the impedance Z₁ at the input port 231 is not equal to thecharacteristic impedance Z₀, the voltage drop V_(x)(t) across the tophalf of the autotransformer 236 is not equal to the voltage drop fromthe mid-point of the autotransformer 236 to ground. Therefore, in this“mismatched input port” case, the voltage drop V_(x)(t) across thebottom half of the autotransformer 236 sets the voltage at the outputport 233 to some proportion of the source voltage V_(S)(t) depending onthe ratio of the impedances Z₁ and Z₀. Thus, even in the ideal case, theisolation degrades and a signal can pass from output port 232 to outputport 233 without suffering severe attenuation.

FIGS. 3A-3D show transfer responses for a simulation of a coaxial cablenetwork based on an ideal hybrid splitter circuit model. The simulatednetwork includes a voltage controlled voltage source with series outputresistor connected to the input port “Port 1” of the splitter over a 50feet length of 75-Ohm coaxial cable to provide a variable impedancedrive to the network. Two additional voltage controlled voltage sourceswith shunt input resistors are connected to the output ports “Port 2”and “Port 3” over 50 ft. lengths of 75-Ohm coaxial cable, respectively,to provide variable impedance output loads for the network. FIGS. 3A-3Dshow the transfer response between ports of the simulated network undera variety of terminating conditions for the source and loads.

FIGS. 3A and 3B show transfer responses with the cable terminationimpedances for all three ports “matched” to the cable characteristicimpedance of 75 Ohms. In the plot of FIG. 3A, showing an input-to-outputresponse, the attenuation in decibels (dB) of the path from Port 1 toPort 2 is nearly flat as a function of frequency over a bandwidth of 0to 30 MHz. Internal splitter power losses (e.g., due to resistive powerdissipation) are minimal in practice and are modeled as 1 dB in thisexample. The nominal total attenuation of around 4 dB is due to thecombination of this internal splitter loss, the dielectric loss of thecoaxial cable (which increases with frequency), and loss due to avoltage divider effect where some power is dissipated in the outputresistor of the source. The simulation models the coaxial cables usingcharacteristics of an RG59 type coaxial cable.

In the plot of FIG. 3B, showing an output-to-output transfer response asa function of frequency, the input port cable termination is set to 74Ohms to simulate the likely conditions of imperfect impedance matchingwhich results in output port isolation that is not infinite. The cabletermination at Port 2 and Port 3 are 75 Ohms. The resulting transferresponse plot shows the high attenuation of the path from Port 2 to Port3 of over 50 dB. The oscillation in the transfer response is due to thechanging impedance transformation properties of the 50 ft. coaxial cablewith changing frequency (according to standard transmission linetheory).

FIG. 3C shows an output-to-output transfer response as a function offrequency with the cable termination impedance for Port 1 set to 250Ohms, for Port 2 set to 5 Ohms, and for Port 3 set to 250 Ohms. Thisconfiguration corresponds to a simple two leaf tree network in which theroot node is terminated with a mismatched high impedance, one leaf nodeis terminated with a mismatched low impedance, and the other leaf nodeis terminated with a mismatched high impedance. As described in moredetail below, in some implementations network devices 112 are configuredto use a low impedance for transmission and a high impedance forreception. The resulting transfer response plot shows the loweredattenuation of the path from Port 2 to Port 3 of around 0 to 10 dB.

FIG. 3D shows an output-to-output transfer response as a function ofinput Port 1 cable termination impedance as it is varied from 5 to 250Ohms. The frequency for the response shown in FIG. 3D is assumed to be15 MNHz. The cable termination impedances of Port 2 and Port 3 are thesame as in the plot of FIG. 3C. The resulting transfer response plotshows the dramatic rise in attenuation (or equivalently the fall intransfer response) of the path from Port 2 to Port 3 that occurs whenthe cable termination impedance at the input Port 1 approaches the75-Ohm characteristic impedance of the transmission line at which thesplitter is designed to have high output port isolation.

Signal Modulation

A coaxial cable network in which one or more are mismatched tends tosuffer from increased passband ripple in the frequency domain andincreased delay spread in the time domain. Both are artifacts caused byreflection of a signal at a mismatched end of a coaxial cabletransmission line. Some high-speed digital communications signalmodulation techniques do not tolerate excessive passband ripple or delayspread.

To achieve robust communication performance in the presence of passbandripple and delay spread, the network devices 112 use OrthogonalFrequency Division Multiplexing (OFDM), also known as Discrete MultiTone (DMT). OFDM is a spread spectrum signal modulation technique inwhich the available bandwidth is subdivided into a number of narrowband,low data rate channels or “carriers.” To obtain high spectralefficiency, the spectra of the carriers are overlapping and orthogonalto each other. Data are transmitted in the form of symbols that have apredetermined duration and encompass some number of carriers. The datatransmitted on these carriers can be modulated in amplitude and/orphase, using modulation schemes such as Binary Phase Shift Key (BPSK),Quadrature Phase Shift Key (QPSK), or m-bit Quadrature AmplitudeModulation (m-QAM).

In OFDM transmission, data are transmitted in the form of OFDM“symbols.” Each symbol has a predetermined time duration or symbol timeT_(s). Each symbol is generated from a superposition of N sinusoidalcarrier waveforms that are orthogonal to each other and form the OFDMcarriers. Each carrier has a peak frequency f_(i) and a phase Φ_(i)measured from the beginning of the symbol. For each of these mutuallyorthogonal carriers, a whole number of periods of the sinusoidalwaveform is contained within the symbol time T_(s). Equivalently, eachcarrier frequency is an integral multiple of a frequency intervalΔf=1/T_(s). The phases Φ_(i) and amplitudes A_(i) of the carrierwaveforms can be independently selected (according to an appropriatemodulation scheme) without affecting the orthogonality of the resultingmodulated waveforms. The carriers occupy a frequency range betweenfrequencies f₁ and f_(N) referred to as the OFDM bandwidth.

Referring to FIG. 4, a communication system 400 includes a transmitter402 for transmitting a signal (e.g., a sequence of OFDM symbols) over acommunication medium 404 to a receiver 406. The transmitter 402 andreceiver 406 can be incorporated into network devices coupled to thecoaxial cable network (e.g., as part of a device transceiver). Thecommunication medium 404 can represent a path from one device to anotherover the coaxial cable network, or a path through another type ofnetwork such as a power line network. Due to their being designed formuch lower frequency transmissions, AC wiring exhibits varying channelcharacteristics at the higher frequencies used for data transmission(e.g., depending on the wiring used and the actual layout). As withmismatched coaxial cable network 100, a power line network exhibitsdistortion due to multipath delay spread. The use of OFDM signals canimprove reliability of communication in coaxial cable networks, powerline networks, or bridged networks including both coaxial cable andpower line sections, as described in more detail below.

At the transmitter 402, modules implementing the PHY layer receive aninput bit stream from a medium access control (MAC) layer. The bitstream is fed into an encoder module 420 to perform processing such asscrambling, error correction coding and interleaving.

The encoded bit stream is fed into a mapping module 422 that takesgroups of data bits (e.g., 1, 2, 3, 4, 6, 8, or 10 bits), depending onthe constellation used for the current symbol (e.g., a BPSK, QPSK,8-QAM, 16-QAM constellation), and maps the data value represented bythose bits onto the corresponding amplitudes of in-phase (I) andquadrature-phase (Q) components of a carrier waveform of the currentsymbol. This results in each data value being associated with acorresponding complex number C_(i)=A_(i) exp(jΦ_(i)) whose real partcorresponds to the I component and whose imaginary part corresponds tothe Q component of a carrier with peak frequency f_(i). Alternatively,any appropriate modulation scheme that associates data values tomodulated carrier waveforms can be used.

The mapping module 422 also determines which of the carrier frequenciesf₁, . . . , f_(N) within the OFDM bandwidth are used by the system 400to transmit information. For example, some carriers that areexperiencing fades can be avoided, and no information is transmitted onthose carriers. Instead, the mapping module 422 uses coherent BPSKmodulated with a binary value from the Pseudo Noise (PN) sequence forthat carrier. For some carriers (e.g., a carrier i=10) that correspondto restricted bands (e.g., an amateur radio band) on a medium 404 thatmay radiate power no energy is transmitted on those carriers (e.g.,A₁₀=0).

An inverse discrete Fourier transform (IDFT) module 424 performs themodulation of the resulting set of N complex numbers (some of which maybe zero for unused carriers) determined by the mapping module 422 onto Northogonal carrier waveforms having peak frequencies f₁, . . . , f_(N).The modulated carriers are combined by IDFT module 424 to form adiscrete time symbol waveform S(n) (for a sampling rate f_(R)), whichcan be written as

$\begin{matrix}{{S(n)} = {\sum\limits_{i = 1}^{N}{A_{i}{\exp\left\lbrack {j\left( {{2{\pi\mathbb{i}}\;{n/N}} + \Phi_{i}} \right)} \right\rbrack}}}} & {{Eq}.\mspace{14mu}(1)}\end{matrix}$

where the time index n goes from 1 to N, A_(i) is the amplitude andΦ_(i) is the phase of the carrier with peak frequency f_(i)=(i/N)f_(R),and j=√−1. In some implementations, the discrete Fourier transformcorresponds to a fast Fourier transform (FFT) in which N is a power of2.

A post-processing module 426 combines a sequence of consecutive(potentially overlapping) symbols into a “symbol set” that can betransmitted as a continuous block over the communication medium 404. Thepost-processing module 426 prepends a preamble to the symbol set thatcan be used for automatic gain control (AGC) and symbol timingsynchronization. To mitigate intersymbol and intercarrier interference(e.g., due to imperfections in the system 400 and/or the communicationmedium 404) the post-processing module 426 can extend each symbol with acyclic prefix that is a copy of the last part of the symbol. Thepost-processing module 426 can also perform other functions such asapplying a pulse shaping window to subsets of symbols within the symbolset (e.g., using a raised cosine window or other type of pulse shapingwindow) and overlapping the symbol subsets.

An Analog Front End (AFE) module 428 couples an analog signal containinga continuous-time (e.g., low-pass filtered) version of the symbol set tothe communication medium 404. The effect of the transmission of thecontinuous-time version of the waveform S(t) over the communicationmedium 404 can be represented by convolution with a function g(τ;t)representing an impulse response of transmission over the communicationmedium. The communication medium 404 may add noise n(t), which may berandom noise and/or narrowband noise emitted by a jammer.

At the receiver 406, modules implementing the PHY layer receive a signalfrom the communication medium 404 and generate a bit stream for the MAClayer. An AFE module 430 operates in conjunction with an Automatic GainControl (AGC) module 432 and a time synchronization module 434 toprovide data and timing information to a discrete Fourier transform(DFT) module 436. After synchronizing and amplifying a received symbolset using its preamble, the receiver 406 demodulates and decodes thesymbols in the symbol set.

After removing the cyclic prefix, the receiver 406 feeds the sampleddiscrete-time symbols into DFT module 436 to extract the sequence of Ncomplex numbers representing the encoded data values (by performing anN-point DFT). Demodulator/Decoder module 438 maps the complex numbersonto the corresponding bit sequences and performs the appropriatedecoding of the bits (including deinterleaving and descrambling).

Any of the modules of the communication system 400 including modules inthe transmitter 402 or receiver 406 can be implemented in hardware,software, or a combination of hardware and software.

Network Interface

FIG. 5A illustrates an exemplary bidirectional AFE module 500 thatserves as a network interface for a network device 112 that incorporatesthe functions of both transmitter 402 and receiver 406. The AFE module500 uses coupling module 502 to receive a signal from the coaxial cable111 to a receiver AFE module 430, and to transmit a signal from atransmitter AFE module 428 into the coaxial cable 111. This approach isa half-duplex approach in which the device 112 is either in a transmitmode or a receive mode at any given time.

FIG. 5B shows circuitry for one implementation of a coupling module 502.The circuitry includes a wideband toroidial transformer 504, transientprotection diodes 506A and 506B, and an F series 75-Ohm female connector508 to accept standard RG59 or RG6 coaxial cable. Terminals from thetransformer 504 form a bidirectional signal interface 510 that includesa differential pair of transmit terminals PL_TXP and PL_TXN from thetransmitter AFE module 428. These transmit terminals optionally includesymmetric resistors with resistance R₀ to set the output impedance andresulting signal level. The signal interface 510 also includes adifferential pair of receive terminals PL_RXP and PL_RXN to connect tothe receiver AFE module 430. The effective input impedance of thenetwork device 112 is selected by setting a resistance in the receiverAFE module 430 to the appropriate value.

Improved communication performance can be achieved when the outputimpedance of a network device 112 driving a signal onto a cable is lessthan the characteristic impedance of the coaxial cable 111.

Some wideband line drivers are operational amplifier circuits withfeedback that achieve very low output impedances (a few Ohms or less).In some systems these drivers are matched to a system characteristicimpedance using a series resistance equal to the system impedance. Avoltage divider is formed by the series matching resistor and the systemload impedance. One half of the driver output potential reaches the loadresulting in 6 dB signal loss for the matched impedance case.

For communication techniques for which this impedance matching is notnecessary (e.g., OFDM) the output impedance of a driver can be reducedto a few Ohms. The resulting loss due to the voltage divider is lessthan the previous case especially when low impedance loads areencountered. The low impedance driver achieves less loss and in somecases gain for many paths through the coaxial cable network 100(relative to the 6 dB loss of a matched impedance driver). For example,an output impedance of about 5 Ohms for a 75-Ohm coaxial cablecharacteristic impedance provided robust performance for signals in the2 to 28 MHz frequency range in a test coaxial cable network.

Improved performance can also be achieved when the input impedance of anetwork device 112 receiving a signal over a cable is larger than thecharacteristic impedance of the coaxial cable 111. In some preferredimplementations, the effective input impedance of the network device 112is selected to be at least 1.2, 2, 3, or 10 times larger depending onthe desired coupling properties. For example, an input impedance ofabout 250 Ohms for a 75-Ohm coaxial cable characteristic impedanceprovided robust performance for signals in the 2 to 28 MHz frequencyrange in a test coaxial cable network.

Network Bridges

A bridge device 116 can use any of a variety of techniques to couplesignals between the coaxial cable network 100 and the secondary network120 depending on the characteristics of the networks. For example, OFDMsignal modulation is well-suited for the nonlinear channelcharacteristics of both the mismatched coaxial cable network 100 and apower line network. A bridge device 116 can couple signals betweencoaxial cable and power line media “passively” without necessarilychanging the signal modulation characteristics. A passive bridge deviceis able to preserve modulation characteristics of a communication signalsuch as the shape of the waveform used to modulate data, and thereforedoes not need to delay a signal for demodulation, buffering, and/orre-modulation.

Alternatively a bridge device 116 can be an “active” device thatdemodulates a signal received over one of the networks and buffers theencoded information for subsequent transmission over the other network.An active bridge device can switch between the networks accessing themone at a time. Alternatively, an active bridge can represent two logicalnetwork nodes with one operating in the first network (e.g., the coaxialcable network) and the other operating in the second network (e.g., apower line network). This type of active bridge device can potentiallycommunicate in both networks at the same time. Both logical nodes insidethe device can be implemented with a single processor and separatephysical interfaces. This active approach introduces a delay in thesignal as it passes through the bridge device 116.

The bridge device 116 can optionally be a simple coupling device thatpasses signals between two networks (passively or actively), or it canbe incorporated into a fully functional network device 112 that servesas an origin and destination for transmitted signals as well as a bridge(passive or active).

In implementations in which the secondary network 120 is a power linecommunication network, the bridge device 116 includes components tofilter out the low-frequency (e.g., 50 Hz or 60 Hz) power waveform, andcomponents to protect against large transient surges in the power line.The communication signal waveform also carries power, however, thevoltage level and corresponding average power of the communicationsignal (e.g., the amplitude of the OFDM symbols) is much smaller thanthat of a typical power waveform with a root-mean-square voltage in therange of 120-240 V.

FIG. 6 shows a passive bridge 600 for bridging coaxial cable and powerline networks in a house. The passive bridge 600 safely couples acommunication signal (e.g., at 2-28 MHz) between the two networks whileblocking the power signal (e.g., at 60 Hz) from crossing form the powerline network to the coaxial cable network. The passive bridge 600includes a wideband coupling transformer 602 that couples a differentialmode signal in either direction between a coaxial cable interface 606(e.g., an F series female coaxial cable connector) and a power lineinterface 608 (e.g., AC power plug prongs). In some implementations thetransformer 602 has a 1:1 turns ratio. Alternatively, the transformer602 can have a different turns ratio to provide an effective change inimpedance. This bidirectional signal coupling enables the coaxial cablenetwork and powerline network be part of the same broadcast domain inwhich the CSMA/CA MAC protocol operates. The transformer 602 also servesto block unintentional common mode energy (noise) while passing thedesired differential mode signal energy. The transformer 602 can befabricated with bifilar turns of wire on a ferrite toroid core. Tripleinsulated Teflon wire is used to provide safety isolation (with a 3 kVbreakdown voltage) between the power line and coaxial cable networks.

The passive bridge 600 includes high-voltage series capacitors 604A and604B (e.g., 0.01 microFrarad capacitors) which act as a high-pass filterto pass the desired high-frequency communication signal and block (orsignificantly attenuate, e.g., by a factor of 10, 100, or more) thelow-frequency power waveform from passing through the transformer to thecoaxial cable network 100. Capacitors 604A and 604B with safe failuremodes can be used to preserve coupler safety in the event of componentfailure. Shunt resistors 612A and 612B (e.g., 200 kOhm resistors)dissipate any residual charge present on the capacitors when the bridge600 is unplugged. A high-voltage varistor 610 maintains a highresistance for voltages within the expected operating range and switchesto a low resistance conducting state to clamp large transient arrivingon the power line that could exceed the breakdown voltage of thecapacitors 604A and 604B. Alternatively, any of a variety oftransient-suppression circuit elements can be used to block (orsignificantly attenuate) voltage transients, including, for example, atransient voltage suppression diode.

FIG. 7 shows an exemplary plastic housing 700 for the components of thepassive bridge 600 with built-in AC power plug prongs 702 as the powerline interface 608. During use, the bridge 600 plugs into an availableAC power outlet in a house. The AC power plug prongs 702 arenon-polarized and may be inserted with either orientation. A length ofcoax cable (e.g., 3 to 12 feet) may be used to connect an F connector704 on the bridge 600 with an F connector port of the coaxial cablenetwork 100.

FIG. 8 shows a hybrid coupler 800 that couples a network device 112 toeither or both of a coaxial cable network and a power line network, andoptionally serves as a bridge between the coaxial cable and power linenetworks. The hybrid coupler 800 includes a wideband couplingtransformer 802 with four isolated windings. The turns ratio istypically unity for all four windings. Triple insulated Teflon wire isused to provide safety isolation (with a 3 kV breakdown voltage) betweenthe power line, coaxial cable, and the low voltage bidirectional signalinterface 804. The signal interface 804 includes a differential pair oftransmit terminals TX_P and TX_N that connect to the output of thetransmitter AFE module 428, and a differential pair of receive terminalsRX_P and RX_N that connect to the input of the receiver AFE module 430.These four lines are low voltage safety isolated connections.

The hybrid coupler 800 includes switches 806A and 806B to select powerline only operation, coaxial cable only operation, or hybrid operationon both power line and coaxial cable media. The power line mediaconnection includes the capacitors 604A and 604B, resistors 612A and612B, the varistor 610, and the power line interface 608, as describedabove. The coaxial cable media connection includes the coaxial cableinterface 606, as described above. The switches 806A and 806B are doublepole single throw switches that make or break the differentialconnections between the coupling transformer 802 and the power line andcoaxial cable media. The switches 806A and 806B can be set at the timeof installation, or alternatively can be controllable via an externalswitch interface.

The power line and coaxial cable media are bridged together (in themanner of the passive bridge 600) when both switches 806A and 806B areclosed. For example, closing both switches allows the network device 112to communicate simultaneously on both the power line and coaxial cablenetworks. Closing both switches in a hybrid coupled network device 112at a first node linked to both networks couples the two networkstogether so that a second node on the power line network can communicatewith a third node on the coaxial cable network through the first node asa bridge.

WORKING EXAMPLE

FIG. 9 shows a plan view of a residential test site 900 showing AC poweroutlets (power line ports PL-1 to PL-7) at which devices connect to apower line network, and coaxial cable ports (coaxial cable ports CX-8 toCX-11) at which devices connect to a coaxial cable network. The coaxialcable network has the topology of a tree network with two 2-waysplitters connected by RG6 type coaxial cable 111. A source port CX-8 isconfigured to interface with a source (or “root”) node of the treenetwork and to distribute a signal to devices connected to the coaxialcable ports CX-9 to CX-11 representing the leaf nodes of the treenetwork. The nominal insertion loss from port CX-8 to port CX-10 or portCX-11 was 7 dB, and the nominal insertion loss from port CX-8 to portCX-9 was 3.5 dB. The AC wiring of the power line network (not shown)forms a shared communication medium such that each power outlet shares abidirectional communication path with every other power outlet.

The signal attenuation representing the port-to-port transfer responsewas measured between all pairs of ports(PL-1 to PL-7, and CX-8 toCX-11). The transfer response was measured in both directions (e.g.,transmitting from port CX-8 to port CX-9, and transmitting from portCX-9 to port CX-8). Since many paths have attenuation that varies withfrequency (e.g., exhibiting peaks and nulls) the average attenuation wascalculated and recorded.

FIG. 10 shows the test setup used to perform the transfer response testmeasurements. A first test node 1002 was coupled to either a coaxialcable port 1004 (one of the 4 ports of the test site 900) or a poweroutlet 1006 (one of the 7 outlets of the test site 900). A second testnode 1008 was coupled to either a coaxial cable port 1010 (one of the 4ports of the test site 900) or a power outlet 1006 (one of the 7 outletsof the test site 900). One of the test nodes was placed in a transmitmode and the other was placed in a receive mode. If the transmittingnode was coupled to a coaxial cable port, then the output impedance ofthe transmitting node was set to a low value of about 5 Ohms. If thereceiving node was coupled to a coaxial cable port, then the outputimpedance of the receiving node was set to a high value of about 250Ohms.

Some of the measurements were performed with the coaxial cable and powerline networks coupled using a passive bridge 600, and some of themeasurements were taken with the coaxial cable and power line networksuncoupled (i.e., with the passive bridge 600 disconnected). In thesetest measurements, when the source port 104 was not participating in themeasurement it remained disconnected (and therefore terminated with amismatched open circuit impedance).

FIGS. 11A and 11B show grids representing the path attenuationmeasurements in which the row corresponds to the transmitting port (PL-1to PL-7, and CX-8 to CX-11) and the column corresponds to the receivingport (PL-1 to PL-7, and CX-8 to CX-11). The shading at the intersectionof a row and column is proportional to the path attenuation. The shadedsquares represent attenuation levels according to the scale 1100. Sincea port does not transmit to itself the diagonal squares (1 to 1, 2 to 2,etc) do not represent attenuation measurements.

The grid in FIG. 11A shows attenuation measurements between all pairs ofports and/or outlets with the passive bridge 600 disconnected such thatthe power line network and the coaxial cable network are uncoupled. Thepower line network connectivity is represented by the lower leftquadrant (rows 1-7, columns 1-7) and the coaxial cable networkconnectivity is represented by the upper right quadrant (rows 8-11,columns 8-11). The average power line network attenuation is about 40 dBwith a wide range of variation. The average coaxial cable networkattenuation (with impedance mismatch) is less than 10 dB. Theattenuation between networks is 60 dB or more (rows 1-7, columns 8-11,and rows 8-11, columns 1-7).

The grid in FIG. 11B shows attenuation measurements between all pairs ofports and/or outlets with the passive bridge 600 connecting the powerline and coaxial cable networks. The average attenuation between powerline outlets remains about the same. The average attenuation between thecoaxial cable ports also shows little change. However, the averageattenuation between the power line and coaxial cable networks is greatlyimproved (i.e., reduced attenuation). The average attenuation levels forthese power line to coaxial cable and coaxial cable to power line paths(rows 1-7, columns 8-11, and rows 8-11, columns 1-7) are similar tothose for power line to power line paths (rows 1-7, columns 1-7), on theorder of 40 dB. These new communication paths provide greaterconvenience and coverage.

Additionally, the communication data rates were measured over these samepaths and the average throughput over a set of paths in various networkconfigurations were calculated, as summarized in Table 1 below.

TABLE 1 Bridge Average Network # Ports # Paths Present ThroughputCoax—Coax 4 12 NO 117.5 mbps  Coax—Coax 4 12 YES 118.2 mbps  PL—PL 7 42NO 72.1 mbps PL—PL 7 42 YES 69.6 mbps PL- Coax 11 56 YES 82.6 mbpsComplete 11 110 YES 78.5 mbps

One set of paths for which the average throughput was measuredcorresponds to the coaxial-to-coaxial paths (rows 8-11, columns 8-11),with and without the passive bridge 600 present. Another set of pathsfor which the average throughput was measured corresponds to the powerline-to-power line paths (rows 1-7, columns 1-7), with and without thepassive bridge 600 present. Another set of paths for which the averagethroughput was measured corresponds to the power line-to-coaxial paths(rows 1-7, columns 8-11, and rows 8-11, columns 1-7), with the passivebridge 600 present. The average throughput was also measured for allpaths (rows 1-11, columns 1-11) with the passive bridge 600 present.

The presence of the passive bridge 600 did not have a large effect onthe average throughput of the existing coaxial-to-coaxial and powerline-to-power line paths, while greatly increasing the total number ofpaths available for communicating in the test site 900.

Many other implementations other than those described above are withinthe invention, which is defined by the following claims.

1. A method for communicating over a coaxial cable network, the methodcomprising: identifying at least one port in the coaxial cable networkthat provides high mutual isolation among nodes of the coaxial cablenetwork when the port is terminated with an impedance that matches acharacteristic impedance of coaxial cable in the coaxial cable network;terminating the identified port with an impedance that is substantiallymismatched with the characteristic impedance of the coaxial cable; andtransmitting a signal from a first node in the network to a second nodein the network; wherein transmitting the signal from the first node tothe second node comprises coupling a signal to the second node with aninput impedance that is substantially mismatched with the characteristicimpedance of the coaxial cable; and wherein the input impedance islarger than about 300% of the characteristic impedance of the coaxialcable.
 2. The method of claim 1, wherein the identified port comprisesan input port to a splitter having at least two output ports that aremutually isolated when the input port is terminated with an impedancethat matches the characteristic impedance of the coaxial cable.
 3. Themethod of claim 2, wherein the splitter comprises a hybrid splitter. 4.The method of claim 1, wherein the identified port is positioned in thenetwork to distribute an incoming signal from a source to terminal nodesof the coaxial cable network.
 5. The method of claim 4, wherein thesource is a cable television feeder cable, a terrestrial antenna, or asatellite dish.
 6. The method of claim 4, wherein terminating theidentified port with the mismatched impedance comprises coupling theincoming signal from the source to the identified port with an outputimpedance that is substantially mismatched with the characteristicimpedance of the coaxial cable.
 7. The method of claim 4, whereinterminating the identified port with the mismatched impedance comprisesuncoupling the source from the identified port.
 8. The method of claim1, wherein transmitting the signal from the first node to the secondnode comprises coupling a signal from the first node with an outputimpedance that is substantially mismatched with the characteristicimpedance of the coaxial cable.
 9. The method of claim 8, wherein theoutput impedance is substantially smaller than the characteristicimpedance of the coaxial cable.
 10. The method of claim 9, wherein theoutput impedance is smaller than about 10% of the characteristicimpedance of the coaxial cable.
 11. The method of claim 1, wherein thecoaxial cable network has a tree topology with the identified port atthe root of the tree.
 12. A coaxial cable network comprising: a sourceport providing an input signal; a coaxial cable coupling the source portto a first splitter; and a plurality of coaxial cables providing aninterface for nodes of the network, at least some of the coaxial cablesbeing coupled to the source port over a path that includes at least onesplitter; wherein at least one splitter port provides high mutualisolation among nodes of the coaxial cable network when the splitterport is terminated with an impedance that matches a characteristicimpedance of coaxial cable in the coaxial cable network; and thesplitter port is terminated with an impedance that is substantiallymismatched with the characteristic impedance of the coaxial cable; andwherein at least a first node in the network is configured to transmit asignal to a second node in the network, where the transmitting comprisescoupling a signal to the second node with an input impedance that issubstantially mismatched with the characteristic impedance of thecoaxial cable; and wherein the input impedance is larger than about 300%of the characteristic impedance of the coaxial cable.
 13. The coaxialcable network of claim 12, wherein the splitter port comprises an inputport to the first splitter, the first splitter having at least twooutput ports that are mutually isolated when the input port isterminated with an impedance that matches the characteristic impedanceof the coaxial cable.
 14. The coaxial cable network of claim 13, whereinthe coaxial cable network has a tree topology with the input port to thefirst splitter at the root of the tree.
 15. The coaxial cable network ofclaim 12, wherein the first splitter is positioned in the network todistribute the incoming signal from the source port to terminal nodes ofthe coaxial cable network.
 16. The coaxial cable network of claim 12,further comprising a node coupled to a coaxial cable interface with anoutput impedance that is substantially mismatched with thecharacteristic impedance of the coaxial cable.
 17. The coaxial cablenetwork of claim 12, further comprising a node coupled to a coaxialcable interface with an input impedance that is substantially mismatchedwith the characteristic impedance of the coaxial cable.